Automatic gain control loop

ABSTRACT

In conventional optical receivers the dynamic range is obtained by using variable gain amplifiers (VGA) with a fixed trans-impedance amplifier (TIA) gain. To overcome the SNR problems inherent in conventional receivers an improved optical receiver comprises an automatic gain control loop for generating at least one gain control signal for controlling gain of both the VGA and the TIA. Ideally, both the resistance and the gain of the TIA are controlled by a gain control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/524,366, filed Jul. 29, 2019, now allowed, which is a continuation ofU.S. patent application Ser. No. 16/200,852, filed Nov. 27, 2018, nowU.S. Pat. No. 10,454,441, which is a continuation of U.S. patentapplication Ser. No. 15/927,537, filed Mar. 21, 2018, now U.S. Pat. No.10,171,057, which are hereby incorporated by reference herein in theirentireties.

TECHNICAL FIELD

The present invention relates to an optical receiver, and in particularto an optical receiver with an analog front-end including an automaticgain control loop.

BACKGROUND

High order modulation schemes, like PAM-m and m-QAM (m>2), are beingused in short reach optical communication links in modern data centersfor future 400 Gb/s links to achieve higher spectral efficiency withrespect to the basic NRZ modulation scheme. Such high order modulationschemes require optical links with strengthened linearity requirement onthe transmitter and receiver to not distort the transmitted signal. FIG.1 illustrates a typical linear optical receiver 1 used in coherentoptical receiver comprising a photo diode (PD) 2, a trans-impedanceamplifier (TIA) 3, and a variable gain amplifier (VGA) 4, which form thereceiver analog front-end (AFE) 5. The optical receiver also comprisesan analog to digital converter (ADC) 6 followed by the digital back end7. The optical signal is received by the PD 2, which generates electriccurrent proportional to the received signal power. The TIA 3 convertsthe PD current to voltage, which is amplified by the VGA 4 to thedesired signal level for the ADC 6. A fixed known signal amplitude isrequired at the input of the ADC 6 to make correct analog to digitalconversion for the following digital back-end 7. The variable gainamplifier 4 is used in the receiver RF chain to control the amplitude ofthe output signal from the AFE 5 that is fed to the ADC 6. The outputsignal amplitude from the VGA 4 is set based on the reference voltage ofthe ADC 6.

Conventionally, linear optical receivers 1 are implemented with anautomatic gain control (AGC) loop 8 to fix the receiver AFE outputamplitude for the following ADC 6, as shown in FIG. 2. The AGC loop 8 isa negative feedback loop that comprises a peak detector 9 and an erroramplifier 11. The amplitude of the voltage signal output from the VGA 4is sensed using the peak detector 9 and compared with a referencevoltage signal (OA) using the error amplifier 11 that drives a gaincontrol signal (GC) of the VGA 4. For large loop DC gain, the AGC loop 8settles when the output voltage of the peak detector 9 equals thereference voltage signal (OA), which is considered as a controlling knopfor the receiver AFE output signal amplitude.

FIG. 3 illustrates a plot of a gain control signal GC for a typical AGCloop 8 versus the input PD current (IPD) at different reference voltage(OA) settings. For the same reference voltage OA signal, the gaincontrol GC signal increases, while the input PD current decreases tomaintain the amplitude of the output voltage signal from the VGA 4constant. The receiver dynamic range is defined as the input PD currentrange that is affordable by the receiver 1 for a fixed output amplitudeas shown in FIG. 3 (between IMIN and IMAX). Moreover, the gain controlGC signal value increases with the reference voltage OA signal for thesame input PD current. Thus, there are a set of gain control GC valuesfor the same input PD current dynamic range depending on the referencevoltage OA signal.

Signals from advanced modulation schemes have high signal to noise ratio(SNR), which must be preserved in the receiver chain until the digitalprocessing, in order to achieve the required bit error rate requirement.Thus, a highly-linear, low-noise optical receiver AFE 5 alongside ahigh-resolution ADC 6 are required in order not to degrade the receivedsignal SNR. Furthermore, a wide bandwidth receiver AFE 5 is required toavoid any inter symbol interference introduced in the received signal.Consequently, high baud rate coherent optical receivers 1 arecharacterized using four main aspects: 1) noise, 2) linearity, 3)bandwidth, and 4) dynamic range.

Optical receivers 1 with low noise and high linearity are required inorder not to degrade the received signal SNR. For small input PD currentlevels, the received signal SNR is dominated by the receiver noise;however, for large input PD current levels, the received signal SNR isdetermined based on the receiver linearity. Typically, the receiver AFEnoise is governed by the front-end TIA 3 while its linearity isdominated by the following VGA 4. Thus, a low noise front-end TIA 3 isrequired to be implemented with a highly linear VGA 4 in the coherentoptical receivers 1.

The dynamic range of linear optical receivers is defined as the ratiobetween the maximum overall trans-impedance gain of the receiver AFE 5to the minimum trans-impedance gain, which translates to the minimum andmaximum photo diode currents that can be amplified by the receiver AFE 5for fixed output signal amplitude. In coherent optical links, thereceived optical power can vary between 10 dBm to −15 dBm, whichtranslates to 5 mA to 150 uA photo diode current assuming 0.5 A/W dioderesponsivity. Conventionally, fixed TIA gain and VGA are utilized in theoptical receiver AFE as shown in FIG. 2. However, using one VGA 4 inoptical receivers is not sufficient for achieving 25 dB dynamic range,as a single VGA 4 provides, at most, a 15 dB dynamic range.

FIG. 4 illustrates a conventional way to implement an optical receiverAFE 15 with a high dynamic range. Two VGA stages 14 a and 14 b areutilized for higher receiver dynamic range. The gain from both VGAs 14 aand 14 b is controlled with the AGC loop 8, such that the gain controlsignal (GC) of both are driven with same error amplifier 11 (as in FIG.3). By utilizing two VGA stages 14 a and 14 b, the achieved receiveroverall dynamic range increases as well as its linearity, which improvesthe received signal SNR for large signal levels. However, thisarchitecture suffers from two main drawbacks: 1) poor noise performance,and 2) bandwidth limitation.

In this architecture, the whole dynamic range of the receiver isobtained by the VGAs 14 a and 14 b; however, the front-end TIA 3 isimplemented with a fixed trans-impedance gain (fixed feedback resistor12). In order to increase receiver linearity, the value of the TIAfeedback resistor 12 is set based on the maximum affordable PD currentthat can be amplified without degrading the VGAs linearity performance.Therefore, a TIA 3 with a feedback resistor 12 having a small value isutilized in this architecture to improve the receiver linearity inexpense of its noise performance, as the TIA noise is inverselyproportional to the value of the resistor 12. Large receiver noisedegrades the received signal SNR specifically for small input PDcurrent. Consequently, this architecture suffers from noise-linearitytradeoff in determining front-end TIA gain, which limits the receivedsignal SNR either at high input levels or small levels. Furthermore,increasing the number of amplifying stages in the RF chain increases thereceiver power consumption as well as it reduces its overall bandwidth.

An object of the present invention is to overcome the shortcomings ofthe prior art by providing an analog front end with an automatic gaincontrol loop for controlling the gain from both the VGA and the TIA.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to an optical receivercomprising:

a photodetector for converting an optical signal into an inputelectrical current signal;

a transimpedance amplifier (TIA) for converting the input electricalcurrent signal into an input voltage signal;

a variable gain amplifier (VGA) for amplifying the input voltage signalto a desired voltage level; and

an automatic gain control loop for generating at least one gain controlsignal for controlling gain of the VGA and the TIA.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof,wherein:

FIG. 1 illustrates a schematic diagram of a conventional opticalreceiver;

FIG. 2 illustrates a schematic diagram of a conventional opticalreceiver with an automatic gain control loop;

FIG. 3 illustrates a plot of photodetector current vs gain control (GC)value;

FIG. 4 illustrates a schematic diagram of a conventional opticalreceiver with a pair of variable gain amplifiers;

FIG. 5 illustrates a schematic diagram of a optical receiver with avariable gain trans-impedance amplifier in accordance with an embodimentof the present invention;

FIG. 6 illustrates a plot of TIA gain control vs TIA gain;

FIGS. 7a and 7b illustrate plots of TIA gain vs TIA gain control, andphotodetector current vs gain control for the device of FIG. 5;

FIG. 8 illustrates a schematic diagram of an optical receiver inaccordance with an embodiment of the present invention;

FIG. 9 illustrates a plot of TIA gain vs TIA gain control, andphotodetector current vs gain control for the device of FIG. 8;

FIG. 10 illustrates a schematic diagram of a trans-impedance amplifierin accordance with an embodiment of the present invention; and

FIG. 11 illustrates a signal conditioning circuit in accordance with theoptical receiver of FIG. 8.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art.

FIG. 5 illustrates an alternative way to implement a high dynamic rangeoptical receiver AFE 25, including a photodetector 21. A variable gainTIA (VGTIA) 23 with a VGA 24 are utilized to increase the receiverdynamic range. Both the VGTIA 23 and the VGA 24 are controlled with anAGC loop 28 by the same gain control signal (GC). The trans-impedancegain of the TIA 23 is controlled by varying the value of a variablefeedback resistor 22.

An automatic gain control (AGC) loop 28 may be used to fix the receiverAFE output amplitude for the following ADC 26 and digital back end 27.The AGC loop 28 may be a negative feedback loop that comprises a peakdetector 29 and an error amplifier 11. The amplitude of the voltagesignal output from the VGA 24 is sensed using the peak detector 29 andcompared with a reference voltage signal (OA) using the error amplifier30 that drives a gain control signal (GC) of the VGA 24. For large loopDC gain, the AGC loop 28 settles when the output voltage of the peakdetector 29 equals the reference voltage signal (OA), which isconsidered as a controlling knop for the receiver AFE output signalamplitude.

FIG. 6 illustrates a plot of front-end TIA gain and noise versus thegain control signal (GCTIA). The solid line plot shows that the TIA gainhas two main states, low gain TIA at low GCTIA values and high gain TIAat high GCTIA values. Low TIA gain is obtained using the variablefeedback resistor 22 with a small feedback resistor value at small GCTIAvalues. TIA noise, shown by the broken line plot, experiences anopposite effect to its gain with GCTIA as the noise increases bydecreasing the GCTIA value, i.e. high noise at the low TIA gain and lowGCTIA values. However, large TIA gain is obtained using the feedbackresistor 22 with a large feedback resistor value at high GCTIA values.Since TIA noise experiences an opposite effect to its gain with GCTIA,the noise is low at the high TIA gain and high GCTIA values. Thetransition region between the low and high gain TIA states is called TIAgain control linear region, at which the TIA gain changes between thetwo gain states linearly with GCTIA signal. The TIA gain control linearregion is confined between GCTIA₀ and GCTIA₁ as shown in FIG. 6.

Front-end VGTIA and VGA gains are controlled using the same GC signal ofthe AGC loop 28 (as in FIG. 5). There are two different ways tocoordinate between the variation of TIA gain and VGA gain with GC signalas shown in FIG. 6. In the first way, the TIA gain control linear regionis mapped to high values of the GC signal as shown in FIG. 7(a). Thisway of coordination helps in improving the overall linearity performanceas the TIA gain is set to the smallest value for the large input PDcurrents for any reference voltage OA; however, this configurationsuffers from bad TIA noise performance for small reference voltage OAvalues, because the TIA feedback resistor 22 is set to a minimum valueand consequently the TIA noise is high.

The other configuration to coordinate the gains between both the TIA 23and the VGA 24 is shown in FIG. 7(b). In this configuration, the TIAgain control linear region is mapped to small values of the GC signal.This way improves the noise performance of the receiver front-end 25 asthe largest value of the feedback resistor 22 is utilized for small PDcurrent irrespective to the OA setting. However, this configurationsuffers from poor linearity performance for high input PD currents atlarge OA values.

With reference to FIG. 8, an embodiment of the present inventioncomprises a linear optical receiver 31 including an AFE 35 with highdynamic range. The optical receiver 31, which may be used in a coherentoptical receiver, comprises a photo diode (PD) 32, a trans-impedanceamplifier (TIA) 33, and a variable gain amplifier (VGA) 34, which formthe receiver analog front-end (AFE) 35. The optical receiver 31 alsocomprises an analog to digital converter (ADC) 36 followed by thedigital back end 37. The optical signal is received by the PD 32, whichgenerates electric current proportional to the received signal power.The TIA 33 converts the PD current signal to an input voltage signal,which is amplified by the VGA 34 to the desired signal level for the ADC36. A fixed known signal amplitude is required at the input of the ADC36 to make correct analog to digital conversion for the followingdigital back-end 37. The VGA 34 is used in the receiver RF chain tocontrol the amplitude of the output voltage signal from the AFE 35 thatis fed to the ADC 36. The output signal amplitude from the VGA 34 is setbased on a reference voltage OA, which is based on known signalamplitudes required by the ADC 36.

An automatic gain control (AGC) loop 38 is provided to fix the outputamplitude of the receiver AFE 35 for the following ADC 36. The AGC loop38 is a negative feedback loop that comprises a peak detector 39 and anerror amplifier 41. The amplitude of the voltage signal output from theVGA 34 is sensed using the peak detector 39 and compared with thereference voltage signal (OA) using the error amplifier 41, which drivesa gain control signal (GC1) to the VGA 34. For large loop DC gain, theAGC loop 38 settles when the output voltage of the peak detector 39equals the reference voltage signal (OA).

The variable gain TIA (VGTIA) 33 and the VGA 34 are utilized to increasethe dynamic range of the optical receiver 30. In the proposedarchitecture, two different gain control signals, e.g. a first gaincontrol signal GC1 and a second gain control signal GC2, may be utilizedfor the VGA 34 and the VGTIA 33, respectively. The first gain controlsignal GC1 is generated using the error amplifier 41 of the AGC loop 38.A signal conditioning circuit 40 is utilized to generate the second gaincontrol signal GC2 for the TIA 33 using the first gain control signalGC1 and the reference voltage OA signal, as illustrated in FIG. 11. Thesignal conditioning circuit 40 may adjust the starting point and theslope of the voltage control line for GC₁ and GC₂, and may include anopamp 60 receiving the OA and GC1 signals and outputting the GC2 signal.The same reference voltage OA signal may be used for both the TIA 33 andthe VGA 34 or different reference voltages may be used.

The transimpedance gain from the TIA 33 may be controlled by varying avalue of a variable feedback resistor 43 and the feed-forward amplifiergain simultaneously. The proposed architecture improves the receivernoise and linearity over wide range of input PD current levels atdifferent reference voltage OA settings. Controlling gain from the VGTIA33 and the VGA 34 with two different control signals, GC1 and GC2,resolves the trade-off between noise and linearity shown in the priorart. The AFE 35 results in a high SNR of the received signalirrespective to its strength, such that the AFE 35 has the best noiseperformance for small input currents from the photodetector (PD) 32;while having the best linearity performance for large PD currents.

As illustrated in FIG. 9, in the proposed embodiment, the gain from thefront-end TIA 33 depends on the second gain control signal GC2 as wellas the reference voltage OA signal. A set of TIA gain controls curvesare generated by shifting the TIA gain control linear region based onthe reference voltage OA signal. Using this technique, the TIA gainvariation is consistent with the input PD current variation, such thatthe TIA variable feedback resistor 43 has the largest value for smallinput PD current and smallest value for large input PD current, whichimproves the receiver noise and linearity performances. Accordingly, thereceiver AFE 35 improves the received signal SNR irrespective to inputPD current level or the OA signal value. Furthermore, utilizing theVGTIA 33 and only the one VGA 34, instead of a fixed gain TIA followedby two VGAs offers wider bandwidth performance with lower powerconsumption.

On the other hand, the proposed receiver AFE 35 may use a shunt feedbackTIA topology to implement the VGTIA 33. Shunt feedback TIA loopstability depends on the feed-forward gain Ao with the value R_(F) ofthe feedback resistor 43. Accordingly, the phase margin of the VGTIA isexpressed as,

$\begin{matrix}{\phi_{m} \approx {\tan^{- 1}\frac{\sqrt{{C_{T}^{2}\omega_{o}^{2}} + \sqrt{{C_{T}^{4}\omega_{o}^{4}} + {C_{T}^{2}\omega_{o}^{2}\frac{4A_{o}^{2}}{R_{F}^{2}}}}}}{\frac{2A}{R_{F}}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where C_(T) is the input node capacitance of the TIA 33 and ω_(o), isthe feed-forward amplifier bandwidth of the TIA 33. The above equationdepicts that the TIA phase margin degrades significantly by reducing thefeedback resistor 43 for constant feed-forward amplifier gain. Thus,implementing the variable gain TIA (VGTIA) 33 with a fixed feed-forwardgain A_(o) has stability issues for small feedback resistor values whichlimits its dynamic range. The proposed receiver AFE 35 may use avariable gain feed-forward amplifier in the front-end VGTIA 33 toimprove its stability by maintaining its phase margin constant byvarying the value of the feedback resistor R_(F) and the feed forwardgain A_(o), simultaneously, ideally with the same ratio (in Eq. 1) andpreferably keeping R_(F)/A_(o) constant.

An example of the TIA 33 of the proposed embodiment is shown in FIG. 10.The TIA 33 comprises a differential TIA with a feed-forward amplifier ofCML stage 51 followed by an emitter follower 52. The feed-forwardamplifier gain A_(o) is controlled by varying the DC current of the CMLstage 51. The gain variation of the feed-forward amplifier is consistentwith the variation of the effective value of the feedback resistors 43and 44. The effective value of the feedback resistance R_(F) iscontrolled by the gain control signal GC_(2 or) GC_(TIA) applied to thegates of control transistors 45 and 46, connected in parallel with thefixed feedback resistors 43 and 44, respectively. At the same time, thegain A₀ of the core amplifier is controlled by a tail current 48. Thetail current 48 is set by the tail current transistor 47, which is alsocontrolled by the gain control signal GC₂ or GC_(TIA.)

The foregoing description of one or more embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

We claim:
 1. An electrical circuit comprising: a transimpedanceamplifier (TIA) comprising: a variable gain feed-forward amplifierconfigured to generate a feed forward gain, and a feedback resistor; anda gain control loop operatively coupled to the variable gainfeed-forward amplifier and configured to vary the feed forward gainthereof.
 2. The electrical circuit according to claim 1 comprising again amplifier electrically coupled to the TIA and configured to amplifyan output signal thereof.
 3. The electrical circuit according to claim 1wherein the gain control loop is further configured to vary the feedbackresistor.
 4. The electrical circuit according to claim 3, wherein thegain control loop is configured to simultaneously vary the feedbackresistor and the feed forward gain so as to keep a ratio of valuesthereof generally constant.
 5. The electrical circuit according to claim3, wherein the feedback resistor comprises a control transistor and afixed feedback resistor connected in parallel, and wherein the gaincontrol loop is coupled to a gate of the control transistor.
 6. Theelectrical circuit according to claim 2, wherein the gain control loopis configured to generate a first gain control signal in dependence on afirst reference voltage for controlling the feed forward gain of thevariable gain feed-forward amplifier.
 7. The electrical circuitaccording to claim 6, wherein the gain control loop includes: a peakdetector capable of measuring an amplitude of an output signal of thegain amplifier; and a device capable of comparing the amplitude of theoutput signal with the first reference voltage.
 8. The electricalcircuit according to claim 6, wherein the gain control loop isconfigured to vary the feedback resistor and the feed forward gain sothat a gain of the TIA varies linearly with the first gain controlsignal.
 9. The electrical circuit according to claim 8, wherein the gaincontrol loop is configured to vary the feedback resistor to have alarger value for a comparatively small input electrical signal to theTIA and a smaller value for a comparatively large input electricalsignal to the TIA.
 10. The electrical circuit according to claim 6,wherein the gain amplifier comprises a variable gain amplifier (VGA),and wherein the gain control loop is configured to control a gain of theVGA.
 11. The electrical circuit according to claim 2, wherein the gaincontrol loop comprises a signal conditioning circuit.
 12. The electricalcircuit according to claim 10, wherein the gain control loop isconfigured to vary the gain of the VGA based on a second referencevoltage.
 13. The electrical circuit according to claim 12, wherein thegain control loop comprises: a peak detector capable of measuring anamplitude of an output signal of the VGA, and an error amplifierconfigured to compare the amplitude to the second reference voltage; andwherein the gain control loop is configured to generate a second gaincontrol signal based at least in part on an output signal of the erroramplifier for controlling at least one of the feedback resistor of theTIA and the gain of the VGA.
 14. The electrical circuit according toclaim 2, further comprising an analog to digital converter (ADC) forconverting an output signal of the gain amplifier, or a signal obtainedtherefrom, into a digital signal.
 15. The electrical circuit accordingto claim 1, further comprising a photodetector configured to generate aninput electrical signal of the TIA in dependence on input light.